(1) Field of the Invention
This invention relates to a pointed, small-sized and highly precise heat-generating device used as a tip heater for molding synthetic resin.
(2) Description of the Prior Art
In the type of pointed heat-generating devices existing presently, there is disclosed a Japanese Patent Laid Open No. 59-180308 registered by the present applicant.
Referring to FIG. 5 and FIG. 6, 1 indicates a projectile-shaped heat-generator main body comprising a cylindrical metal outer tube 2 and a pointed conical heat-generating front part 3 provided in the front area. This pointed heat-generating front part 3 is provided with a through hole 4 on its central axis. A heat generating means 5, for example, of nichrome wire is placed through hole 4 until one end of the wire reaches the tip of the heat-generating front part 3 and is welded thereto. An alloy part X of antifriction material of the weld filler material is shaped to conform to the shape of the pointed heat-generating front part 3.
The heat-generating means or heater 5 has a coiled heat-generating part 5c and extension wires 5a, 5b extending from its ends. The beater 5 utilizes a nichrome wire of 0.1-0.6 mm in diameter, formed in a coil with a winding diameter of about 1.3-2.0 mm. 6 indicates a ceramic column inserted and fixed locally to the coiled heat-generating part 5c of said heater, effectively insulating the heater from other members as needed. The heater 5 extends close to and fixed to the tip of the through hole 4 of the projectile-shaped heat-generating main body. That is, to the tip of the conical heat-generating front part 3. 7 indicates one extension of the heater 5 pierced longitudinally along through hole 4 of the heat-generating main body #1. Further, the other extension line 5b of the heater 5, similarly is thermally welded to the end of the conical heat-generating front part 3, forms a part of the alloy part X of antifriction material which is antifrictional in the same degree as the main body #1 and forming a grounds the end of the heater 5.
As constructed above, the heater 5 is heated by electric current passing between the extension line 7 of the heater 5 and the projectile heat-generating mainbody 1. The heat moves immediately and responsively to the conical heat-generating front part 3. By radiating outwardly from the conical heat-generating front part 3, the heat thermally melts the thermoplastic synthetic resin in a gate area leading to a cavity. Cooling and solidification of the thermoplastic synthetic resin in the gate area leading to the cavity can be achieved by stopping the electric current to the heater 5, thereby stopping immediately heat generation in the heat-generating part 5c, heat transmission to the conical heat-generating front part 3, and radiation of heat from said part 3 to the thermoplastic synthetic resin.
Thus it is possible for us to manipulate injection molding by passing electricity intermittently to the heater 5, thereby intermittently heating and melting and cooling and soidifying the thermoplastic synthetic resin in the vicinity of a gate, and accordingly opening and closing the gate.
Further, although the preceding embodiment is constructed with a body heater 8 coiled in the inside of the heat-generating mainbody 1 and insulated on both sides by the insulation pipes 9 and 10, other types of injection molding devices exist without a body heater 8 in the heat-generating mainbody 1. These devices may substitute a coiled heater in the area surrounding the runner (not shown) and thereby heat and fuse the thermoplastic synthetic resin in the same manner.
In the preceding embodiment, a fine diameter wire, placed as close as possible to the extremity, is used for the heater 5 and the coiled heat-generating part 5c has the same diameter and same pitch along its entire length. This makes the distance between the tapered peripheral surface Y and the heater 5 gradually shorter toward the tip and gradually longer toward to the rear portion.
Accordingly, the prior art device fails to provide a quick response. Heat conveyed from the coiled heat-generating part 5c of the heater 5 to the tapered surface Y is variable depending upon the point on the tapered surface Y. Thus the time required to attain the necessary temperature throughout the entire pointed heat-generating front part 3 is delayed.
Further, although the precedent embodiment permits a saving in electric consumption by using a fine heater coiled in a small diameter and a smaller electric controller, it is possible to reduce electric consumption further and to make the controller smaller by improving responsiveness of the heater even if the wire diameter is somewhat enlarged.